In a time-of-flight (TOF) range sensor that acquires a range image through use of the flight time of light, a MOS structure is used for controlling a potential just under a gate electrode in a vertical direction against a principal surface of a semiconductor chip, in which the MOS structure is merged. For example, as recited in PTL 1, a CMOS range-finding element and a TOF image sensor including the CMOS range-finding elements are proposed. The CMOS range-finding element includes an n-type charge-generation buried-region buried in a p-type semiconductor layer, charge-transfer buried-regions, charge read-out buried regions, an insulator covering the layer and the regions, transfer electrodes, which are arranged on the insulator and transfer signal charges to the charge-transfer buried-regions, and read-out gate electrodes, which are arranged on the insulator and transfer signal charges to the charge read-out buried regions. The charge-generation buried-region receives a light pulse, and optical signals are photoelectrically converted into signal charges in the semiconductor layer just under the charge-generation buried-region. Then, based on a distribution ratio of the charges accumulated in the charge-transfer buried-regions, a distance from a target is measured.
With the CMOS range-finding element and the TOF image sensor, which includes the CMOS range-finding elements, there are concerns over such problem that noise and a dark current may be caused. The noise and the dark current are ascribablr to, for example, interface defects and interface states just under the transfer electrode of the CMOS range-finding element. Further, in a case where the transfer electrodes described in PTL 1 are used, actually, it is difficult to control a potential-gradient over a long distance, and it is impossible to maintain an electric field substantially constant over a long charge-transfer channel. Thus, in the photoelectric-conversion element such as a range-finding element having a long charge-transfer channel, there is caused such inconvenience that carriers are stopped in the middle of the charge-transfer channel and expected performance is not easily obtained.
Moreover, in recent year, in a field of biomedical science, a time-resolution image sensor has been used more widely. Among techniques adopting the time-resolution image sensor is fluorescence lifetime imaging microscopy (FLIM) for measuring a time period of attenuation of fluorescence, or the fluorescence lifetime, by measuring intensity of fluorescence, the fluorescence is excited by irradiating light to molecules in cells. It is expected that the application of FLIM provide a considerable impact on a field of medical science and preventive medicine.
As recited in PTL 2, the inventors of the present invention have already proposed a four-tap lateral electric field (LEF) control photoelectric-conversion element capable of acquiring continuous time-resolution components with four short time-windows at low noise while maintaining a high signal/noise ratio (S/N ratio), the photoelectric-conversion element disclosed in PTL 2 includes four charge-accumulation regions provided at quadruple positions symmetric with respect to a center position of a light-receiving area and field-control electrode (gate electrode) pairs provided to both sides of paths to the respective charge-accumulation regions. The charges generated through the photoelectric conversion are transported while destinations of the charges are sequentially set to the first charge-accumulation region to the fourth charge-accumulation region. A time-window is set to a period of a subnanosecond, and single-shot measurement is performed with triple or quadruple time-windows at the same time. Subsequently, the third or fourth incoming light corresponding to the timing for the triple or quadruple time-windows are delayed as a whole to perform measurement in a measurement time range directly after the time-window of the first time. Those actions are repeated at several times and joined. In this manner, time resolution of a subnanoseconds required for the fluorescence lifetime measurement and a measurement time range of several nanoseconds can be achieved.
According to the technology recited in PTL 2, a potential profile to maintain an electric field substantially constant is easily controlled over the long charge-transfer channel, and the signal charges are transported to the plurality of regions through the long charge-transfer channel at a high speed with satisfactory symmetry. Moreover, there can be provided the photoelectric-conversion element, which can avoid a problem of occurrence of noise and a dark current caused by, for example, interface defects and interface states in the interface at the semiconductor surface and a problem of reduction in transport speed, and the solid-state image sensor with low noise, high resolution, and a high response speed in which the plurality of photoelectric-conversion elements are arrayed. However, in a case of a single shot, there is a problem in that the four-tap photoelectric-conversion element can acquire only three or four components among a plurality of fluorescence time resolution components. Further, in a case where acquisition is performed by repeating a single shot, there is a problem in that a total measurement time is increased.